1. Field of the Invention
This invention relates to elongate electrical devices comprising two spaced-apart insulating jackets and means for limiting the damage which can result from water permeating along the device between the jackets, e.g., along a metallic braid between the jackets.
2. Introduction to the Invention
Elongate electrical devices of many kinds, in particular heaters and sensors, have been described in prior publications and in co-pending, commonly assigned, patent applications. Particularly useful are self-regulating heaters which comprise two or more elongate electrodes having a plurality of heating elements connected in parallel with each other between the electrodes; for example, heaters comprising two or more electrodes embedded in a PTC conductive polymer element are commercially available and give excellent results. Other heaters comprise a resistive heating element composed of a metal. Reference may be made for example to U.S. Pat. No. 4,242,573.
Many elongate electrical devices comprise inner and outer insulating jackets which are spaced apart from each other, typically by a metallic braid which provides an electrical shield as well as added resistance to physical abuse. A disadvantage of this type of device is that if water enters the space between the jackets, e.g., as a result of damage to the outer jacket, the water can wick down the heater between the jackets. This can occur as a result of capillary action even when there is no hydrostatic pressure. If the water reaches either end of the device, there is a risk that the device will malfunction, e.g., by shorting the two (or more) electrodes of a parallel heater. So long as the water does not reach either end of the device, it does not usually have any adverse effect on the performance of the device because the inner jacket excludes the water from the electrically active part of the device.